Field of the Invention
This invention relates generally to an imaging system, and specifically to an improved migration imaging system utilizing an imaging member having a thermoplastic imaging surface layer.
Description of the Prior Art
Within the art of electrophotography are imaging processes and system- which involve the migration of pigmented particles in a liquid or softenable medium to achieve an imagewise pattern or an image-receiving member. In electrophoretic and photoelectrophoretic recording systems, a liquid suspension of photoconductive particles disposed in a dielectric material between a pair of planar electrodes is imagewise exposed to actinic radiation. Exposed particles migrate to one electrode, and unexposed particles migrate to the other. In this manner, positive and negative images are produced on the respective electrodes. In mediums that are not liquid or permeable at room temperature, particle migration can be facilitated by the ,softening of the medium by the application of heat or solvents.
Another type of migration imaging system utilizes a solid migration imaging member which comprises a transparent conductive substrate, a layer of softenable material overlying the substrate, and a uniform layer of photoconductive marking material deposited atop the softenable layer. A latent image is formed on the particle layer by electrostatically charging this layer and then exposing it to an imagewise pattern of light to discharge selected portions of the layer. The entire softenable layer is then uniformly heated to render it permeable to the photoconductive particles on top of it. The non-exposed portions of the particle layer, i.e., those portions that retain a charge after the light exposure will migrate into the softened layer by electrostatic forces. One example of such an imaging process is disclosed in U.S. Pat. No. 4,883,731, issued to Tam et al. While this imaging system appears to be technically viable to overcome some of the problems associated with photoelectrophoresis. It is disadvantageous in that high power is required to soften the entire softenable layer. Moreover, it requires the use of photoconductive marking particles. Also, the images formed in the solid imaging members processed according to the foregoing approaches have been found to lack the image contrast, gray scale accuracy, and sharp resolution required in high-resolution image reproduction- A simpler and more efficient imaging system would be desirable.
In the published International Patent Application WO 88/04237, filed by Polaroid Corporation, there is disclosed a thermal imaging medium which includes a support sheet having a surface layer of a heat-liquifiable material and an overlying layer of a pigmented particulate or porous material. A pressure-sensitive adhesive layer overlies the particulate layer. The liquifiable material is imagewise exposed to heat to cause it to flow by capillary action into the particulate or porous layer. With cooling, the imaged areas of the substance are thereby retained by the particulate or porous material on the support sheet. The adhesive layer is then peeled away, causing the unexposed areas of the particulate layer to break from the exposed areas and be carried with the adhesive layer. The support sheet retains the exposed pattern.
A problem with the above process is that the fracturing between exposed and unexposed areas of the particulate layer can be uneven or irregular. Moreover, the heat-liquifiable material is expected to flow only into a certain volume of the pigmented particulate layer, but the flow is not restricted. The liquified material can flow laterally into a volume that is adjacent the heated area and which is not part of the image to be reproduced. The perimeter of an image component (a dot, for example) would be greater than intended. As a result, image quality can be degraded.
In general, prior art adhesive transfer and migration imaging systems are also materials-intensive and thus are costly to operate. This is especially so in systems which consume materials that are not provided in a simple, easy-to-use, and inexpensive form.
Significant waste products are generated in many of the above-described adhesive systems. Adhesive transfer systems generate discarded peel-away films which are usually not reusable. Proper disposal of such waste is inconvenient and increases operating costs.
Migration imaging and adhesive transfer processes have, therefore, not been favored for image reproduction in a number of applications, especially in high-resolution or high-speed printing systems.